Swell control



Feb. 26, 1963 T. B. GIBBS r-:TAL 3,078,753

swELL. CONTROL Filed Dec. 14, 1959 J0 f J// J/ TOME AMM/nf@ f/f S14/ffl.f (oA/720A '/4- G15/VERA 70K las vf ym @Zwam zgia Wim Patented Feb. 26,1963 3,078,753 SWELL CNTRL Thomas B. Gibbs, Delavan, and William H.Krug, .lanesville, Wis., assignors to Gibbs Manufacturing & ResearchCorporation, a corporation of Wisconsin Filed Dec. 14, 1959, Ser. No.859,424 2 Claims. (Cl. Sli- 1.27)

This invention relates to a swell control for an electronic musicalinstrument and more particularly with an improved electronic swellcontrol circuit.

In an electronic musical instrument, as an electronic organ, it isundesirable to use a simple voltage divider or potentiometer to controlthe volume of the ultimate signal from the system as such elementsintroduce noise, are subject to wear, and in many cases must bespecially manufactured to maintain a desired relationship of volumelevel to the deiiection or other actuation of a control member. A swellcontrol has been proposed in which a variable impedance device, as avacuum tube, is shunted across the signal channel of the audio amplifierand its impedance is manually varied, as by varying the light shining ona photoelectric cell connected with the control tube. The variation inthe shunt impedance presented to the signal channel causes a resultingchange in the amplitude of the audio signal. It has been found, however,that control results in a shift of the D.C. potential of the anode ofthe Vacuum tube which is coupled from the variable impedance element tothe signal channel with resultant interference in the nature of an audiofrequency superimposed upon the signal.

A principal object of this invention is the provision in the swellcontrol circuit of a variable impedance element that compensates for theeifect of D.C. anode potential shift of the swell control circuit andthe resultant audible disturbance to the signal.

One feature of the invention is that the variable irnpedance element isconnected in the anode circuit of the control tube and essentially formsa parallel path connected in shunt with the control tube, compensatingfor D.C. anode potential shift of the control tube.

Another feature .is the provision in the circuit of a feedback networkwhich cancels hum from the control tube at low swell levels. A furtherfeature is that the control tube is responsive to a photoelectric cellconnected in the grid circuit thereof with a capacitor connected in thephotocell circuit, eliminating high frequency interference.

Further features and advantages of the invention will readily beapparent from the following specification and from the drawings, inwhich:

FIGURE l is a block diagram of a musical instrument system; and y FIGURE2 is a schematic circuit diagram of an embodiment of the invention.

In certain electronic musical instruments the control of the volume ofthe produced tones is extrem-ely important in determining the quality ofthe instrument. For example, in an electronic organ, the swell controlmust correspond accurately in operation with the operation of the swellcontrol in a pipe organ, having a specitic relation between the angulardeection `of the swell control pedal and the acoustic level of thegenerated tones. It is desirable to eliminate from such a circuit anyphysically variable element as a variable resistor or potentiometer, inwhich it is difficult to control the exact operating characteristics andwhich has a tendency to wear unevenly and change characteristics withage, and to introduce noise interference in the circuit. This is done invarious circuits utilizing a variable impedance control tube connectedwith the signal channel. However, as

outlined above, merely substituting a variable impedance tube for apotentiometer is not the whole solution of the problem. In certaincircuits of the aforementioned application, the variable plateresistance of the tube is essentially connected in parallel with theplate dropping or load resistor, shunted across the signal channel. Acertain portion of the audio signal in the signal channel is coupled tothe swell control tube resulting in a variation of the tube plateresistance at an audio rate, and a corresponding variation in thevoltage level at the plate. This in effect amounts to a distortion ofthe audio signal with a resultant high frequency variation in the signallevel which is objectionable. The circuit of the present inventioneliminates this problem by shunting the variable impedance tube with anelement which has a resistance variation in response .to the signalvoltage that compensates `for the change of the plate resistance of thetube.

In FIGURE 1 the basic elements of an electronic musical instrument areillustrated in block form. A tone generator 10 produces or generateselectrical signals corresponding with the desired musical tone Iand mayyfor example utilize a light scanning system. The tone representingsignal is transmitted through a signal channel 11 to an amplifier 12which has a suitable speaker system 13 connected with its output.Connected in shunt with signal channel 11 is a swell control 14 whicheffectively varies the amplitude of the signal and thus controls thevolume of the sound from speaker system 13.

A speciiic embodiment of the invention will be described in connectionwith the schematic circuit of FIG- URE 2 and values and Itypes will beassigned to the various elements of the circuit. It will be understoodthat this specific circuit is intended to illustrate an operativeembodiment of the invention and that many changes and modifications maybe made by those skilled in lthe art.

The output of tone generator 10, which comprises an electrical signalrepresenting the desired musical tones, is coupled through capacitor 18,0.01 pif., Ito the control grid of an amplifier 19, one-halt of a l2AX7dual triode. The control grid is returned to a reference potential orground 20 through resistor Z1, 470,000 ohms; and the cathode circuitincludes an unbypassed bias resistor 2v2, 2700 ohms. The plate ofamplifier 19 is connected through anode load resistor 23, 330,000 ohms,with a source 24, of positive potential. The amplified tone representingsignal from ampliiier 19 is connected through a parallelresistance-capacitance circuit including resistor 25, 1 megohm', shuntedby capacitor 26, 0.00047 uf., yforming a part of the signal handlingchannel 11.

Swell control 14 includes a triode 29, one-half of a 12AT7, connected inshunt with signal handling channel 11 through capacitor 30, 0.05 nf. Apositive bias is applied to Kthe cathode of variable impedance tube 29through a voltage divider connected with positive source 24 includingresistor 31, 150,000 ohms, connected in series with potentiometer 32,15,000 ohms, connected between the cathode and ground 20. Potentiometer32 is shunted by capacitor 33, 4.0 uf. With no signal applied to thegrid of variable impedance tube 29, it is for all practical purposes cutoff and presents a high impedance to signal channel 11. The grid circuitincludes a pair of resistors 34, 680,000 ohms, and 35, l megohm,connected in series and returned to ground 20. A manually operable swellcontrol is provided by a photoelectric cell 36 which has its plateconnected with positive source 24 and its cathode returned to groundthrough resistor 35 in the circuit of the control grid of variableimpedance tube 29. Light from lamp 37 is directed past a mask member 3Smounted to swell pedal 39 to the cathode of photoelectric cell 36. Withfull light shining on the photocell it conducts heavily, applying apositive potential to the control grid of variable impedance tube 29reducing its impedance and establishing a condition of low swell orminimum volume. As pedal 39 is depressed mask 38 progressively cuts olimore and more of the light from lamp 37 directed toward photocell ,36reducing the current tlow through the photocell and thus reducing thepositive potential applied to the control grid of variable impedancetube 29. With the liUht fully masked, there is no conduction through thephotocell and the impedance of tube 29 is a maximum, corresponding withthe condition of full swell in the instrument.

As pointed out above a portion of the tone representing signal in signalchannel 11 is coupled through capacitor 30 to the plate of variableimpedance tube 29. This audio signal at the plate tends to cause thecurrent through tube 29 and thus the impedance thereof to vary at therate of the audio signal resutling in an undesired distortion in thesignal channel.

In order to eliminate this condition, the plate circuit of variableimpedance tube 29 includes an element 42 the resistance of which variesinversely with the voltage applied to it, as a variable resistor elementof a silicon carbide material, manufactured and sold by General Electricunder the trademark Thyrite Varistor. For an analysis of the alternatingcurrent operation of the circuit, the plate circuit impedance element 42may be considered as paralleling the plate resistance of the variableimpedance tube 29. It will be seen that when the signal voltage at theplate of tube 29 goes positive the current ilowing through the tubeincreases, causing an apparent decrease in the plate resistance. At thesame time, the resistance of variable impedance element 42 increases, asa result of the decreased voltage across it when the audio signalincreases toward the positive potential of B supply 24. Accordingly, thechange in the impedance of the plate circuit elements compensates forthe change in the plate resistance of the variable impedance tube andthe entire shunt circuit presents a substantially constant impedance tothe signal channel. When the audio signal voltage goes negative, theoperation is reversed, the plate resistance of tube 29 increasing andthe resistance of variable element' 42 decreasing. A resistor 43,470,000 ohms, is connected in shunt with variable impedance element 42and their relative values are such that the parallel combination closelyapproximates an inverse exponential resistance relationship with theplate resistance of tube 29. The variable impedance element 42 hascharacteristics such that its resistance may cover the range from 20,000ohms to 2 megohms, yfor the circuit described herein.

The substantial elimination of the eect of distortion of the audiosignal in the variable impedance circuit permits the use of a feedbackcapacitor 44 connected between the plate and control grid of tube 29which has suflicient capacity to eliminate substantially all hum andnoise developed within the tube 29. Specifically, this capacitor hasbeen increased from 0.0.02 nf, to a value of 0.007 uf.

Furthermore, capacitor 45, 0.02 pf., is connected across one megohmresistor 35 in the circuit of photocell 36. This reduces anyinterference from low frequency current variations in the photocellcircuit, or hum pickup from the interconnecting cables.

While we have shown and described certain embodiments of our invention,it is to be understood that it is capable of many modifications. Changestherefore, in the construction and arrangement may be made withoutdeparting from the spirit and scope of the invention as disclosed in theappended claims.

We claim:

l. In an electronic musical instrument having a source of tonerepresenting signals and a signal handling channel connected to a sourceof potential, swell control means comprising: a circuit including avariable impedance electron tube having a plate circuit and a gridcircuit having a bias potential applied thereto; a rst reactive elementcoupling said plate circuit to said signal channel, a second reactiveelement coupling said grid circuit to the junction of said plate circuitand said iirst reactive clement; manual control means for effecting achange in the bias voltage applied to said grid for atiecting theimpedance of said tube to vary the level of the signal in said channel;and a variable impedance element connected to said plate circuit at saidjunction and to said source of potential whereby it is in partial shuntwith said first reactive element and said signal channel, said variableimpedance element having an impedance which varies inversely with thevoltage applied thereto for preventing signal changes in said channelfrom affecting the output of said tube.

2. In an electronic musical instrument having a source of tonerepresenting signals and a signal handling channel connected through arst resistance element -to a source of potential, swell control meanscomprising: a circuit includ-ing a variable impedance electron tubehaving a plate circuit and a grid circuit having a bias potentialapplied thereto; a first capacitor connecting said plate circuit tosaid. signal channel; a second capacitor connecting said grid circuit tosaid plate circuit at the junction of said plate circuit with said iirstcapacitor; manual control means for effecting a change in the bias onsaid grid circuit of said tube to vary the level of the signal in saidchannel; and a voltage sensitive resistance element whose resistancevaries inversely with voltage connected at said junction and betweensaidplate circuit and said source of potential in shunt with said rstcapacitor and said lirst resistance element to prevent changes in signallevel appearing in said channel from affecting said tube.

References Cited in the file of this patent UNITED STATES PATENTS1,690,224 Gent Nov. 6, 1928 2,561,747 Merrill July 24, 1951 2,695,386Schmidt Nov. 23, 1954 2,712,040 Heytow June 28, 1955 2,838,716 SpiererJune 10, 1958 2,854,606 Spiegel Sept. 30, 1958

1. IN AN ELECTRONIC MUSICAL INSTRUMENT HAVING A SOURCE OF TONEREPRESENTING SIGNALS AND A SIGNAL HANDLING CHANNEL CONNECTED TO A SOURCEOF POTENTIAL, SWELL CONTROL MEANS COMPRISING: A CIRCUIT INCLUDING AVARIABLE IMPEDANCE ELECTRON TUBE HAVING A PLATE CIRCUIT AND A GRIDCIRCUIT HAVING A BIAS POTENTIAL APPLIED THERETO; A FIRST REACTIVEELEMENT COUPLING SAID PLATE CIRCUIT TO SAID SIGNAL CHANNEL A SECONDREACTIVE ELEMENT COUPLING SAID GRID CIRCUIT TO THE JUNCTION OF SAIDPLATE CIRCUIT AND SAID FIRST REACTIVE ELEMENT; MANUAL CONTROL MEANS FOREFFECTING A CHANGE IN THE BIAS VOLTAGE APPLIED TO SAID GRID FORAFFECTING THE IMPEDANCE OF SAID TUBE TO VARY THE LEVEL OF THE SIGNAL INSAID